Actually yes. I bought a brand new -discounted old stock- Iphone 4s for my mum near the end of the ios 8 cycle. The day before we installed ios 9 on it, it had okay performance and good battery life. Following the update to ios9 the performance went to complete shit. (the battery remained usable for 2 more years after, but it was not a good experience for her)
On IOS there is not much point in using Firefox, because it’s just an UI shell for Safari with crippled functionality at that. I hope EU law will make real Firefox a possibility.